The number of older Americans is increasing rapidly and global ambient temperatures are predicted to rise in the coming decades. This represents a significant challenge, as older individuals have a reduced tolerance to high ambient temperatures and are therefore at an increased risk of hyperthermia and heat-related illness, injury, and mortality. Both physiological and psychological states influence the health and safety of an individual during exposure to such adverse conditions. Importantly, cognitive changes during hyperthermia often precede physiological impairments, and thus the increased susceptibility to hyperthermia-induced morbidity and mortality in older individuals may be attributed, in part, to hyperthermia-induced alterations in cognitive functioning. However, any relationships between aging, heat stress, and cognition remain unknown. To that end, Aim 1 of this project will examine the effects of healthy aging and hyperthermia on cognitive function. Specifically, we will test the hypothesis that cognitive decrements during hyperthermia are accentuated with healthy aging as compared to younger individuals. This will be accomplished by evaluating cognitive function within the domains of memory, attention, and executive functioning, when internal body temperature is normal (i.e., normothermia) and elevated (i.e., hyperthermia). Aim 2 of this project will investigate a factor that may modify the relationship between heat stress and cognitive function in the aforementioned groups. Specifically, we will test the hypothesis that dehydration impairs cognitive function with hyperthermia, but to a greater extent with healthy aging. This will be accomplished by assessing indices of cognitive function in younger and older subjects during both normothermia and hyperthermia while dehydrated and after rehydration. The knowledge attained from this project will lead to a greater understanding of the effects of hyperthermia, with and without dehydration, and aging on indices of cognitive function. This information will assist the elderly, their clinicians and caregivers, towards reducing the prevalence of heat-related illness, injuries, and death in this vulnerable population. Moreover, these findings will serve as a benchmark from which subsequent studies could be performed. Such studies may include an investigation into the interactive effects of heat stress and aging in individuals with existing cognitive impairments (as it may be that heat stress causes further cognitive impairments in this at risk population), as well as evaluating methods to enhance cognitive functioning during hyperthermia (e.g., heat acclimatization, etc.).